Excitonic near resonance-enhancement of shift currents in monolayer NbOCl2

The shift current is a static nonlinear photocurrent exhibited by crystals lacking center-of-inversion symmetry. Such photocurrents have been widely studied for their potential photovoltaic applications and have potential to go beyond the Shockley-Queisser limit. As such, an accurate theoretical description of the phenomenon is essential for material design. Density functional theory is often utilized in first principles calculations of the shift current, treating the electrons independently. In 2D materials however, the reduced screening results in signficant many-body interactions and photo-excitations are more accurately described with excitons. We have developed a theoretical framework which captures these excitonic effects using many-body perturbation theory within the framework of nonlinear optical response. We derive an expression for the exciton shift current from the static nonlinear response. The many-body excited states are obtained using the GW-BSE formalism by solving the Bethe-Salpeter Equation with quasi-particle energies. Using the many-body excited states, we then compute the excitonic shift current. We show that there is a large enhancement in the shift current when considering many-body interactions, mainly due to the near-resonance condition between two near-degenerate excited states.